This invention relates to xcex1-olefin random copolymer fibers and, more particularly, to such fibers and processes for their preparation from propylene xcex1-olefin random copolymers manufactured using a metallocene catalyst.
Thermoplastic olefin polymers, such as linear polyethylene, polypropylene, and olefin copolymers, such as propylene-ethylene copolymers, are conveniently formed in continuous loop-type polymerization reactors and thermoformed to arrive at granules or pellets of the polymers. For example, polypropylene and propylene-ethylene copolymers are polymerized in continuous polymerization reactors in which the monomer stream is introduced into a reactor and circulated with an appropriate catalyst to produce the olefin homopolymer or copolymer. The polymer is withdrawn from the catalyst reactor and subjected to appropriate processing steps and then extruded as a thermoplastic mass through an extruder and die mechanism to produce the polymer as a raw material in particulate form, usually as pellets or granules. The polymer particles are ultimately heated and processed in the formation of the desired end products.
Polypropylene and propylene copolymers, as used in various applications involving production of films, fibers, and similar products, are thermo-processed and shaped or oriented by uni-directional or bi-directional stresses. Such polymers are thermoplastic crystalline polymers. Isotactic polypropylene is conventionally used in the production of fibers in which the polypropylene is heated and then extruded through one or more dies to produce a fiber preform which is processed by a spinning and drawing operation to produce the desired fiber product.
Isotactic poly-xcex1-olefins traditionally have been catalyzed by well-known multi-site catalysts including Ziegler-Natta type catalysts such as titanium chloride. While such catalysts are useful for producing resins or polymers of xcex1-olefins, including polypropylene and propylene-ethylene random copolymers, they produce polymers with relatively broad molecular weight distributions or polydispersity which include significant fractions of polymer material with both higher and lower molecular weight than the average or nominal molecular weight of the polyolefin polymer. For example, U.S. Pat. No. 4,298,718 to Mayr et al., U.S. Pat. No. 4,560,735 to Fujishita and U.S. Pat. No. 5,318,734 to Kozulla disclose the formation of fibers by heating, extruding, melt spinning, and drawing from polypropylene produced by titanium tetrachloride-based isotactic polypropylene. Particularly, as disclosed in the patent to Kozulla, the preferred isotactic polypropylene for use in forming such fibers has a relatively broad molecular weight distribution (xe2x80x9cMWDxe2x80x9d), as determined by the ratio of the weight average molecular weight (xe2x80x9cMwxe2x80x9d) to the number average molecular weight (xe2x80x9cMnxe2x80x9d) of about 5.5 or above. Preferably, as disclosed in the Kozulla patent, the molecular weight distribution, Mw/Mn, is at least 7.
The high molecular weight fraction found in such Ziegler-Natta reactor-grade isotactic polymers causes processing difficulties for the maker of polypropylene fibrous or fiber-containing products. As explained in U.S. Pat. No. 6,010,588, the high molecular weight fraction contributes significantly to the melt strength of the molten polymer, diminishing the processibility of the polymer. Some of the processing problems involve the need for higher processing temperatures necessary to reduce the inherent melt strength and viscosity and cause the higher molecular weight chains to move. This requires higher energy input to move the polymer through the extruder or other processing equipment. High melt strength also leads to difficulty in forcing the molten resin through a small fiber-forming orifice. Within that restriction, the high molecular weight molecules cause significant drag and diminish flow. Those same molecules also cause significant die swelling of the polymer fibril upon its exit from the fiber-forming orifice due to their inherent tendency toward elastic response with recovery of their conformational bulk. Along with these processing difficulties for fiber manufacturers, the fibers resulting from traditionally produced polypropylene tend to be thick, due to the melt strength of the molten resin. Such fibers lead to formation of fairly coarse fabrics which lack xe2x80x9cgivexe2x80x9d, limiting their use in garments and other applications where a pleasant feel or xe2x80x9chandxe2x80x9d is desirable.
One solution for reducing xe2x80x9cboardinessxe2x80x9d and increasing xe2x80x9csoftnessxe2x80x9d and xe2x80x9cgivexe2x80x9d of fabrics made from polyolefin fiber has been copolymerization of ethylene with propylene to make random copolymers. Small amounts of ethylene monomer are added in a reacting medium comprising propylene and a Zeigler-Natta catalyst capable of randomly incorporating the ethylene monomer into the macromolecule chain, reducing overall crystallinity and rigidity of the macromolecule. Propylene-ethylene random copolymers, because of their lower crystallinity and rigidity, are preferred over homopolymer isotactic polypropylene in fiber and fabric applications that require enhanced softness.
However, like the Ziegler-Natta isotactic polypropylene polymers, the Ziegler-Natta propylene-ethylene random copolymers have fiber processing difficulties. Further, there has been inability of existing fiber and fabric processes to economically draw fine diameter fibers from conventional high ethylene content random copolymers, in particular random copolymers having an ethylene content greater than about 3% by weight. In addition, as explained in U.S. Pat. No. 5,994,482, random copolyers having an ethylene content greater than about 5% by weight generally have not been feasibly produced in liquid reactor or hybrid reactor technologies. Liquid and hybrid reactor systems account for the most part of polypropylene manufacturing capacity worldwide. In a liquid reactor system, the liquid hydrocarbon solubilizes the atactic portion of the polymer, the level of which is enhanced by the high incidence of ethylene monomer in the polymer chain. The atactic material is tacky and creates flowability problems in the downstream equipment as soon as the liquid hydrocarbon is vaporized. Above an ethylene content of about 5% by weight, tacky copolymer granules agglomerate and/or stick to the metal walls of the process equipment.
The processing difficulties described above respecting Zeigler-Natta polymers and copolymers led to development of post-reactor treatment of Ziegler-Natta polymers to enhance processability. Most of these post-formation or post-reactor processes involve some sort of molecular chain scission of the polymer molecules, normally accomplished through the treatment of polyolefins, particularly, polypropylene, with heat and oxygen, or a source of free radicals such as organic peroxides. When organic peroxides are mixed with polypropylene in the melt phase, the polymer is caused to degrade to a narrower molecular weight distribution (xe2x80x9cMWDxe2x80x9d) and lower average molecular weight (xe2x80x9cMwxe2x80x9d) and exhibits a higher melt flow rate (xe2x80x9cMFRxe2x80x9d). The Mw of the visbroken polyolefin is determined by the MFX test (ASTM D1238, Condition L). MFR is a characteristic well known in the art and is reported as grams/10 minutes or dg/min, at 230xc2x0 C. The Mw of a visbroken polyolefin determines the level of melt viscosity and the ultimate desirable physical properties of the fiber. Basically, since a higher MFR flows more melted polymer through an orifice, a lower Mw polymer is more easily melt spun. Most melt spinning is at MFR""s exceeding 35 dg/min.
Degradation of polypropylene polymer to a lower average Mw and a narrower MWD dan the starting material has been termed xe2x80x9cvisbreakingxe2x80x9d the polyolefin. The presence of the organic peroxides in the polypropylene resin results in what is known as xe2x80x9ccontrolled rheologyxe2x80x9d or xe2x80x9cCRxe2x80x9d resin. A peroxide of choice in the polypropylene art in the production of CR polypropylene resins is 2,5-dimethyl-2,5-bis(t-butylperoxy) hexane, available from ATOCHEM, Organic Peroxides Division, Buffalo, N.Y., as Lupersol 101. The post-reactor treatment involving oxidative scission offers several benefits for fiber makers, including reduced overall viscosity and improved flowability, with shifted molecular weight distribution, reduced nominal molecular weight, and significantly reduced fractions of high molecular weight species.
However, post reactor visbreaking of polypropylene and propylene-ethylene random copolymers is not without its drawbacks, for it also results in increases in the fraction of low molecular weight species in the polymer. The lower molecular weight species tend to become volatile during melt processing. This volatility causes difficulty such as an apparent smoking from the material at high temperature when it is not contained, as when it exits a spinning die. The volatility of the low molecular weight fraction also tends to lead to a blooming or surface imperfection on the finished fibers after they are drawn, due to the pitting and cracking which may be caused as the low molecular weight species volatilize. The visbreaking step adds to the expense of the production process. This not only increases costs but also complicates the process of polyolefin resin production for the polymer producer.
In light of the complications caused for both the polymer producer and the end user of the Ziegler-Natta polyolefin in making fiber, improvements in the manufacture of isotactic propylene as reactor-grade materials were sought, and have been made recently, using single site metallocene catalysts. These produce an isotactic polypropylene having a narrow molecular weight distribution that eliminates the problems associated with high melt strengths of the reactor grade Ziegler-Natta isotactic polyolefins, yet they can still have the nominal molecular weight of the post-reactor oxidatively degraded products.
For the convenience herein, various polymers are sometimes identified by abbreviations, as follows:
Metallocene catalysts that produce m-iPP and m-iPE RCP are disclosed in U.S. Pat. Nos. 4,794,096, 4,975,403 and 6,117,957, incorporated herein by reference. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene.
Working to achieve fibers of high strength and softer hand exploiting the narrower molecular weight distributions and nominal molecular weighs of controlled rheology polymers afforded by metallocene catalysis, the art has employed high melt spinning draw speeds, in excess of 2000 m/min (meters per minute) and draw ratios of at least 3 in melt spinning equipment. For example, U.S. Pat. No. 6,010,588, describes the manufacture of m-iPP using rac-dimethylsilanediylbis (2-methyl-4,5-benzo-indenyl) zirconium dichloride with alumoxane as a supported metallocene catalyst. Fibers formed from an m-iPP polymer were prepared, according to U.S. Pat. No. 6,010,588, as spun partially oriented yarns by mechanical take-up of the fiber bundle from a 232xc2x0 C. extruded melt, and were drawn from the melt by an axially spinning unheated godet at 1000, 1500, 2000, 2500, and 3300 m/min. The tenacity of the m-iPP fibers (from resin of melt flow rates of 40, 51 and 68, col. 14, line 1, and Table 1, lines 15-33) exceeded that of both a reactor grade Ziegler-Natta isotactic polypropylene (herein, a xe2x80x9cRx ZN-iPPxe2x80x9d) having a MFR of 35, and a visbroken controlled rheology Ziegler-Natta isotactic polypropylene (herein, a xe2x80x9cCR ZN-iPPxe2x80x9d) having a MFR of 33.
U.S. Pat No. 6,010,588 explains these results, saying of the drawing stress imparted to melt formed fiber, that xe2x80x9cas greater force is applied after the fiber is melt-formed, the tenacity of the single-site catalyst produced ployolefin fibers increases markedly. This is easily seen through recognition of the fact that as the take-up rate increased, the fiber diameter decreases and a greater degree of strain is imparted to the fiber. It is apparent that the test example fibers have noticeably higher tenacities at the higher take-up rates than do either of the control examples.xe2x80x9d The highest tenacity achieved in U.S. Pat. No. 6,010,588 with an m-iPP was with the 40 MFR resin at take up rates of 3300 m/min, giving 4.38 g/den (grams per denier); at the slower draw rate of 1500 m/min, tenacity was less than 3 (i.e. 2.75 g/den). U.S. Pat. No. 6,010,588 indicates that by high speed stretching of m-iPP, one can form smaller diameter, therefore softer, fibers, that have higher tenacity: xe2x80x9cThis newly discovered trait of fibers formed of reactor-grade, metallocene-type produced polymer therefore offers the ability to form smaller fibers requiring less material, which are softer due to their greater flexibility, and which are stronger, yet still may be produced at higher rates; a tremendous set of advantages for any fiber producer.xe2x80x9d
In U.S. Pat. No. 6,146,758, a m-iPP was produced in one instance with dimethyl silyl bis(2-methyl indenyl) zirconiom dichloride and was comparison tested with a commercially available m-iPP believed produced with a bridged bis(indenyl) ligand of enantiomorphic configuration. The m-iPP""s were melt spun into fibers and compared to fibers melt spun from a Rx ZN-iPP. Spinning was performed at a melt temperature of 230xc2x0 C. for the Rx ZN-iPP and at 195xc2x0 C. for the m-iPP polymers. The draw speed was initially at 2000 m/min and increased, in increments of 500 m/min through 4000 m/min while maintaining the draw ratio constant at 3:1. The tenacities of the two m-iPP polymers increased with draw speed (one better than the other) whereas the tenacity of the Rx ZN-iPP decreased with draw speed. The highest m-iPP tenacity was about 4.5 g/denier, achieved at a draw speed of 3000 m/min.
While the tenacities achieved from m-iPP at the high draw speeds exemplified by U.S. Pat. Nos. 6,010,588 and 6,146,758 are impressive, not all fiber processors are able to operate their melt spinning operation at such high draw speeds and draw ratios to obtain fine diameter, high tenacity m-iPP fibers.
Another approach to making fine fibers in order to make soft fabrics having good hand has been to increase the ethylene content of a resin above the limits practically available for liquid and hybrid reactor systems, and make alloys of propylene-ethylene copolymers, as exemplified by U.S. Pat. No. 5,994,482. This patent compares the tenacities obtained with fibers from the patented alloys to tenacities of fibers from melt spun 3% and 5% controlled rheology Ziegler-Natta isotactic propylene-ethylene copolymers (herein, a xe2x80x9cCR ZN-iPE RCPxe2x80x9d) having a MFR of about 33, and shows that the tenacities for the CR ZN-iPE RCP are less than 3.5 grams/denier at a draw speed of 2000 m/min, which is either not much higher or is less than the tenacities achieved with the m-iPP fibers reported in U.S. Pat. No. 6,010,588 (MFR""s of 40 and up) and U.S. Pat. No. 6,146,758 at the same draw speeds. The copolymers exemplified in U.S. Pat. No. 5,994,482 are produced using Ziegler-Natta catalysts, but mention is made (col. 10, lines 49-56) that a metallocene catalyst would be another suitable method of making the copolymers, since it would allow the production of a copolymer alloy having a MFR in the range of from about 35 to about 2000 g/10 minutes with a very narrow MWD, eliminating the need for post reactor oxidative degradation of the alloy.
Also taking the alloy or mixture approach are U.S. Pat. Nos. 5,455,305 and 5,874,505. U.S. Pat. No. 5,455,305, discloses a mixture of a syndiotactic propylene homopolymer and an isotactic propylene homopolymer that is melt spun into fibers at a draw ratio of about 3.8:1, and states that a random copolymer of propylene and an xcex1-olefin selected from ethylene and C4-C8 xcex1-olefins may be used instead of the isotactic propylene homopolymer. U.S. Pat. No. 5,874,505 discloses a mixture of 20 to 97 wt.% metallocene produced isotactic polypropylene homopolymer and 5 to 80 wt. % of an xcex1-olefin copolymer produced by a metallocene catalyst and comprising 10 to 90 wt. % of one xcex1-olefin and 90 to 10 wt. % of another xcex1-olefin, exemplifying, however, only propylene-ethylene copolymers, apparently in the only random copolymer exemplified, of 35 mol % ethylene, assuming copolymerization proportional to feed rates.
Disclosing metallocene catalyzed copolymers of propylene and another xcex1-olefin are U.S. Pat. Nos. 5,959,046, 5,516,866, and 5,763,080. U.S. Pat. No. 5,959,046 discloses a rac-diphenylsilyl-bis{1-(2,7-dimethyl-4-isopropylindenyl)}zirconium dichloride and a rac-dimethylsilyl-bis{1-(2,7-dimethyl-4-isopropylindenyl)}zirconium dichloride catalyzed copolymerization of propylene and ethylene to manufacture propylene ethylene copolymers tested for film and sealant use in which ethylene content ranged from 2.9 mol % to 27 mol %. U.S. Pat. No. 5,516,866 describes crystalline copolymers of propylene with from 2 to 6 mol percent of ethylene or 2 to 10 mol % butene-1, with a low melting point and a limited solubility in xylene at 25xc2x0 C., prepared using metallocene catalysts obtained from stereorigid and chiral compounds of zirconium, and methylalumoxanic compounds. The suggested use was film; no fibers were made or taught made from these compositions. U.S. Pat. No. 5,763,080 discloses fibers produced from metallocene catalyzed copolymers of propylene and 0.2 to 6 mol percent of C5 and higher xcex1-olefins (in particular, 4-methyl-1-pentene, 1-hexene or 1-octene) and fiber bundles comprising a fiber made from the copolymer and a propylene homopolymer, for use in manufacture of spun bonded and nonwoven fabrics. The copolymer provided a lower melting temperature to facilitate spin bonding the more crystalline isotactic propylene homopolymer.
From the foregoing, it is apparent that the art has emphasized high draw speeds and high draw ratios and use of polymers or copolymers having a MFR 35 or higher for melt spinning formation of fibers to obtain fiber properties having high strength and soft hand. At draw speeds above 2000 and draw ratios around 3 or more, none of the approaches of the prior art addresses the problem of producing high tenacity fibers at low melt spinning draw speeds, much less producing a high tenacity fibers at low processing speeds and making a fiber having soft hand.
We have invented new elongated fibers and processes for forming them, such fibers being made from metallocene catalyzed isotactic propylene {acute over (xcex1)}-olefin random copolymers having a melt flow rate of less than 35, The fibers not only have a soft hand, but rather astonishingly, have a very high tenacity, exceeding even that of a metallocene isotactic propylene homopolymer. We use the abbreviation xe2x80x9cm-iPAO RCPxe2x80x9d to indicate the copolymers making up the fibers of this invention (the xe2x80x9cmxe2x80x9d means metallocene catalyzed, the xe2x80x9cixe2x80x9d means isotactic, the xe2x80x9cPxe2x80x9d means propylene, the xe2x80x9cAOxe2x80x9d means ethylene and the C4-C8 {acute over (xcex1)}-olefins, and RCP means random copolymer). When speaking only of a metallocene catalyzed propylene ethylene random copolymer, we use the abbreviation xe2x80x9cm-iPE RCPxe2x80x9d in which xe2x80x9cExe2x80x9d stands for ethylene.
Surprisingly, we have found that m-iPE RCP fibers having these properties of soft hand and high tenacity can be produced at low draw speeds, less than 2000 m/min., and at low draw ratios of about 5 and less, suitably from about 1.5:1 to about 5:1.
The new fibers of this invention are made from m-iPAO RCP copolymers that have melt flow rates less than 35. It is surprising these copolymers are useful for melt spinning to get fibers having a soft hand and high tenacity, for the reason that controlled rheology resins, such as the CR ZN-iPP and CR ZN-iPE RCP resins commonly used for melt spinning fibers, have MFR""s of about 35 and higher, yet do not give the desired qualities these new m-iPAO RCP copolymer fibers possess. It is counter-intuitive in melt spinning to use high molecular weight polymers (polymers having low MFR), for the reasons explained above, and expect to get a product having soft hand and high tenacity, yet remarkably, excellent melt spinning results can be obtained with the metallocene catalyzed copolymers we describe, and at low draw rates and draw ratios, contrary to the direction the fiber art is going with metallocene catalyzed propylene homopolymers (xe2x80x9cm-iPWxe2x80x9d).
In accordance with our invention, we provide an elongated fiber product comprising a drawn metallocene catalyzed propylene-ethylene (or other {acute over (xcex1)}-olefin with a carbon number range in the C4-C8) random copolymer fiber having a C2, C4, C5, C6, C7 or C8 {acute over (xcex1)}-olefin monomer content of from fractional (less than 1), for example about 0.2, to about 10 mol %, preferably 5 mol % or less, more preferably 3 mol % or less, and a MFR of from fractional to less than about 35, suitably about 30 or less, preferably more than 5 (dg/min at 230xc2x0 C.), and capable of being drawn at a draw speed of less tan 2000 m/min, yet having very high tenacity. The fiber is prepared by spinning and drawing at a draw speed suitably about 1500, suitably less, around 1000 m/min, and a draw ratio within the range of 1.5-5:1 preferably at least 2:1, suitably 3:1) and is further characterized by having a tenacity of about 3.5 grams per denier and higher.
In a further aspect of the invention there is provided a process for the production of m-iPAO RCP fibers. In carrying out the process, there is provided an m-iPAO RCP produced by the polymerization of propylene and a C2, C4, C5, C6, C7 or C8 xcex1-olefin monomer in the presence of a metallocene catalyst system comprising a bridged chiral and stereorigid cyclopentadienyl or substituted cyclopentadienyl ligand structure of a transitional metal selected from group 4b, 5b, or 6b metals of the Periodic Table of Elements and having a MFR less than 35, suitably about 30 or less. The copolymer is heated to a molten state and extruded to form a fiber preform. The preform is subjected to spinning at a spinning speed of at least 300 meters per minutes and subsequent drawing at a speed of up to about 1500 meters per minute to provide a draw ratio of at least 1.5 up to 5, preferably 2 and more preferably 3, to produce a continuous m-iPAO RCP fiber having a tenacity of about 3.5 grams per denier and higher.
In yet a further embodiment of the invention, there is provided a process for the production of propylene-ethylene (or other xcex1-olefin with a carbon number range in the C4-C8) random copolymer fibers in which the draw speed and/or the draw ratio can be varied to produce fibers of different mechanical properties. In this aspect of the invention, there is provided an m-iPAO RCP having a MFR less than about 35 produced by the copolymerization of polypropylene and ethylene (or other xcex1-olefin with a carbon number range in the C4-C8) in the presence of an isospecific metallocene catalyst characterized as having a bridged bis(indenyl) ligand in which the indenyl ligand is an enantiomorphic and may be substituted or unsubstituted. The m-iPAO RCP is heated to a molten state and extruded to produce a fiber preform which is then spun at a spinning speed of at least 300, preferably 500 meters per minute or more and subsequently drawn at a spinning speed of up to about 1,500 meters per minute at a draw ratio of at least 1.5, up to about 5, preferably in the range 2-4, most preferably 3, to provide a continuous fiber of a desired physical characteristic. The process involves continuing to provide an m-iPAO RCP copolymer produced by the copolymerization of polypropylene and ethylene (or other xcex1-olefin with a carbon number range in the C4-C8) in the presence of an isospecific metallocene catalyst and heating the polymer to produce a fiber preform which is subjected to spinning under a spinning speed of at least 300, preferably 500 meters per minute or more and subsequently drawn at a spinning speed of up to about 1,500 meters per minute at a draw ratio of at least 1.5, up to about 5, preferably in the range 2-4, most preferably 3.